Generally, a high-grade steel bar product is manufactured by blooming a continuously cast slab having a large rectangular cross section (a so-called bloom) to make a steel billet, and further rolling the billet to make a steel bar product or a wire product. Continuously cast slabs sometimes have defects such as center segregation and center porosity at a final stage of solidification. If the slabs to be materials of high-grade steel bar products have such defects, the properties of the high-grade steel bar products degrade. Therefore, it is important to make such defects not appear in the slabs in continuously casting.
As typical methods of decreasing center segregation and center porosity, a method of carrying out reduction on a slab in a state of having an unsolidified portion inside (hereinafter sometimes referred to as “rolling reduction method in unsolidified state”) and a method of carrying out reduction on a slab completely solidified to the inside (hereinafter sometimes referred to as “rolling reduction method after complete solidification”). According to the rolling reduction method in unsolidified state, it is possible to decrease center segregation, since molten steel in which segregation elements which form an unsolidified portion inside the slab are condensed can be discharged to the upstream side of the casting direction. In addition, according to the rolling reduction method in unsolidified state and the rolling reduction method after complete solidification, it is possible to bond by press and decrease center porosity.
In order to decrease center segregation and center porosity of a slab having a rectangular cross section (hereinafter sometimes referred to as “rectangular slab”) by carrying out reduction on the slab by means of simple cylindrical rolls (hereinafter sometimes referred to as “flat rolls”), a large reduction amount is required to increase the reduction penetration degree to the central part of the slab. Here, the term “reduction penetration degree” refers to the level of concentration of reduction to the central part of the slab. The larger the reduction penetration degree is, the larger the ratio of the deformation amount of the central part of the slab in the reducing direction to the deformation amount of the surface layer part is. According to the difference in the temperature and deformation resistance between the surface layer and the central part in the reducing direction of the slab, the actual reduction amount of the surface layer part of the slab is different from the actual reduction amount of the central part in the reducing direction. The reduction penetration degree also differs depending on the temperature and deformation resistance.
However, if the reduction amount is large, the possibility of the occurrence of internal cracks of the slab increases especially when the slab is reduced in an unsolidified state, and cracks also tend to occur on a non-reduction surface, as shown in FIG. 1 described below. Therefore, there is a problem of having a high possibility that the properties and quality of steel bar products are impaired.
FIG. 1 is a view showing a state of occurrence of cracks on a non-reduction surface in a case where a rectangular slab is reduced by flat rolls. In a case where a rectangular slab 100 is reduced by the flat rolls 101 with the rolling reduction method in unsolidified state or the rolling reduction method after complete solidification, a reduction stress occurs over the entire reduction surface of the rectangular slab 100 and a buckling deformation occurs on the non-reduction surface, whereby a short side part of the slab projects. Therefore, the reduction penetration degree decreases. Further, a tensile strain occurs on the non-reduction surface due to the projecting deformation of the short side part of the slab. In a case where the reduction amount is large, sometimes a crack 102 in the casting direction occurs on the non-reduction surface of the rectangular slab 100, originated from the tensile strain of the surface of the slab.
Against the problem, Patent Literature 1 suggests a method of increasing the reduction penetration degree of an unsolidified portion of a rectangular slab, by reducing only a part of the slab facing the unsolidified portion at a predetermined reduction ratio by means of convex rolls, to increase the reduction penetration degree of the unsolidified portion. The convex roll includes a convex part having a large diameter locally provided for a central part in the width direction of a flat roll.